Tuesday, May 11, 2010

Upgrading the Dynaco SCA-80 integrated amplifier

Some things in life are always worth doing, while others require a furrowed brow. Upgrading a Dynaco SCA-80Q amplifier doesn't exactly define common sense as the design is antiquated at best. However my Ikea record/bookshelf has narrow 13" slots and I already had a matching Dynaco AF-6 tuner that fits nicely. A $25 Ebay win soon had a dusty and decrepit '80Q' in my hands.

Featuring the Dynaco PAT-4 preamplifier and ST-80 amplifier all in one chassis, the 'Q' version added a primitive 'quadraphonic' sound with the use of a single 10 ohm resistors. Looking at the schematic of the SCA-80Q reveals a single common power supply, no regulation, a simple (almost tube-like) topology, and a plethora of electrolytic coupling capacitors. Surely much of the associated harshness must come from the aged parts and poor power supply design.

An article from Van Alstine pointed the way to bypassing the tone controls and a few dollars at Digikey and Ebay soon had some parts speeding my way.

1) Bypass the tone controls by snipping out the tone control wires that lead to numbers 13-18 on the preamp PCBs. On the PCBs, tie leads 13-15 together. See page 29 of the 1986 Audio Basics newletter.

2) On the preamp PCBs, replace the six 50uF/25V Elna electrolytics with 10uF/100V film types. I used some low budget metallized units. I also replace every other low voltage electrolytic as these can be troublesome. Make sure to also replace the two grey electrolytics coming off the PCB boards, located behind the volume control, as these are from the phono stage output. This whole capacitor swapping process is easier if the PCBs are removed from the unit. Of course this requires marking the location of the old wires and some deft use of the soldering iron.

3) Replace 5000/80V power supply capacitor with a 6800uF unit. Replace the four 400 ohm / 7W units on the back power supply PCB with four 390 / 10W units. On the stock unit, this board is known to start on fire, so I kept the new resistors directly off of it.

4) Replace the two side mounted 5000F/80V speaker coupling capacitors. I used some nice modern 4700uF Panasonic units bypassed with some 4.7uF/250V caps from my junk box.

5) The white wire wrapped around the speaker coupling capacitors are actually small valued chokes. I bought a smaller packaged pair from an Ebay seller who specializes in Dynaco parts and mounted them on some terminal strips.

6) I decided to regulate the preamp section by using a simple 7824 regulator. Electronics 123 has a nice like PCB to accomplish this but the 7824 still needs to be bought from Digikey.  This required the changing of the original 4.7k dropper resistor to a 400 ohm / 10W unit. I also changed the phono section power supply to come from the regulated supply and I used a 1.2K / 2W dropping resistor.

Due to laziness and the number of wires to desolder from the board, I did not tackle the amplifier PCBs yet. There are still some electrolytics that need replacing and I have an idea of improving the negative feedback as the output caps and chokes are not included in the loop.

So how does the amplifier sound? Quite modern now. With my KEF Q60s, much of the harshness is gone and only some faint residual 'solid-state' upper midrange hardness can be heard. I was surprised how dynamic the amplifier sounds now as rhythm gels better than my trusty Dynaco PAS-3X preamplifier and McIntosh 250 amplifier. The tube / solid state combination does sound smoother and warmer, but lacks the impact and speed of the refurbished Dynaco SCA-80Q.

Note: for a more detailed upgrade, check out the latter posts on the Dynaco PAT-4 and ST-80 upgrades.